1. Field of the Invention
The present invention relates AC source voltage sensing and current sensing in a synchronous rectifier, and, more particularly, to such sensing in a full bridge synchronous rectification with PFC (power factor correction).
2. Background Information
FIG. 1 is a prior art schematic for a basic rectification, boost circuit that converts an AC (alternating current) input voltage 2 into a DC (direct current) output voltage 4. This circuit has two stages, the first being the diode full wave rectifier composed of D1-D4, and the second being a boost inductor Lb, boost switch Qb, boost diode Db, and an output filter capacitor, Cf. Qb is an N-channel MOSFET where a positive voltage between the gate 10 and the source 12 will turn on Qb and present a low impedance path for current to flow from the drain 14 to the source 12. As shown, the MOSFET Qb has an internal body diode from source to drain 16, that can carry current when the source is at a higher voltage than the drain.
The diodes D1-D4 constitute a full wave rectifier bridge providing a rectified sinewave voltage 12 at the left end of inductor Lb. The power MOSFET Qb is turned on and off by the average current controller 6 at a switching frequency much higher than the AC mains input voltage. Typical switching frequencies are in the range of 40 kHz to 100 kHz. The average current controller pulse width modulates (PWM) the conduction of Qb to force the fundamental current in Lb to be similar in shape to the rectified voltage 12. For example, when the anode of D1 is driven positive with respect to the cathode of D4 by the AC input voltage, the boost inductor Lb is charged (current builds up) through D1, Qb, and D4 back to the AC input 2. When Qb is turned off, the voltage at the anode of Db rises until Lb discharges through Db, and load 8 and the parallel capacitor Cf; and D4 back to the AC source 2. Db prevents the capacitor Cf from discharging back through Qb when it is turned on. The output voltage 4 will be regulated at some DC level, typically 400 Vdc for 120/240Vac AC mains input. On the next AC mains half cycle, Lb is charged via D3, Lb, Qb and D2 back to the AC voltage source. Again when Qb is turned off, Lb discharges through Db and load 8 and parallel capacitor Cf and regulates the output voltage 4 at some level.
The prior art controller 6 of FIG. 1 is designed to vary the d(t) signal pulse width such that the current ird(t) 10 is proportional to the rectified voltage vrd(t). If it is precisely proportional, the power factor would be 1.0, but in practice the power factor will only approach 1.0. As discussed below, that correction works to make the load on the AC voltage source be resistive. In such a case, the current load will be proportional to (i.e., sinusoidal and in phase with) the AC voltage source 2.
In these prior art circuits shown in FIG. 1, the voltage vrd(t) is sensed at the cathodes of D1 and D3, and the load current 10 may be sensed with a current transformer or by sensing the voltage drop across a series resistor.
U.S. Pat. No. 6,738,274 B2 ('274) is directed to a switch-mode power supply where circuit losses are reduced and power factor is corrected. However, in this patent the current sensing and voltage sensing are not shown or discussed in any detail. If resistors and 50/60 Hz transformers are used, as in the prior art, many of the advantages of this patent will be unfulfilled. A series current sensing resistor dissipates power and 50/60 Hz line voltage sensing transformers are lossy and physically bulky. The prior art current sensing via a resistor typically must use an IGBT (an insulated gate bipolar transistor) and separate anti-parallel diodes D10, D12 FIG. 2. The generation of the control signal, see FIG. 5 of the '274 patent, is not detailed in this patent. However, as mentioned above, the design of logic circuitry for the controller or use of a large scale IC (integrated circuit) computer to generate such control signals are known to those skilled in the art.
The operation of the circuit of FIG. 2 is similar to that of FIG. 1, except the PFC function is integrated with the rectification function. The controller operates the IGBT switches Q1 and Q2 so that the load on the AC input appears resistive. The boost inductor, Lb, in this example, is divided into two windings typically on a common core. During the AC input half-cycle when the AC voltage polarity is as labeled in FIG. 2, the charging of the inductor Lb is via Q1, and the discharge is controlled by turning on and off Q1. During this cycle the discharging of the inductor is accomplished via D1, Cf, the load and D12. During the alternate AC input half cycle the discharge path is via D2, the load and D10. The contents of the controller may be a processor that drives Q1 and Q2 to achieve a PFC near unity—that is the load on the AC input appears to be resistive.
Still referring to FIG. 2, a 50/60 Hz AC line transformer provides an isolated representation vrd(t) of the input AC signal to the controller 7. The secondary of this transformer is full-wave rectified to provide a signal proportional to the absolute value of the AC input. The transformer is necessary to elimate the common mode voltages that exist between the AC input and the controller (minus output) common. The current ird(t) drawn from the AC input signal may be sensed from a transformer 9 or from a series resistor Rs. The sensed current is input to the controller 7. The controller outputs switching frequency PWM d(t) signals, in response to the vrd(t) and ird(t), that drives the gates of Q1 and Q2 in order to make the load on the AC input be resistive.
The circuitry to generate these control signals for the circuits in FIGS. 1, 2, 3 and 4a is well within the capability of those skilled in the art. The controller circuitry will include low voltage (+12-15V DC) power supply and programmable digital electronics to generate the signal and the timing shown in the diagrams.
Notice, also, that the circuit of FIG. 1 has two stages, the rectifier diodes and then the power factor correction stage, while the rectification and power factor control are manifest within a single stage in FIG. 2. In FIGS. 2 and 3 a 50/60 Hz transformers are used to sense the AC input voltage. These transformers are big, bulky, lossy and expensive. The transformer is also not able to operate under the conditions of a DC input voltage which may occur for some broad range input applications. An example would be the application of a DC battery voltage to provide UPS (Uninterruptible Power System) features.
PF is the ratio of real power to apparent power and is expressed as a decimal running from 0 to 1.0. Real power is measured for AC signals as the time integral of (volts as a function of time) multiplied by (amps as a function of time). That is
      Real    ⁢                              ⁢                            ⁢    Power    =            1      T        ·                  ∫        0        T            ⁢                                    V            ⁡                          (              t              )                                ·                      I            ⁡                          (              t              )                                      ⁢                              ⅆ            t                    .                    Apparent power or Volt*Amperes is the product of RMS voltage times RMS current. Displacement PF traditionally is expressed in terms derived from the phase difference between a sinusoidal voltage and a sinusoidal current with PF equal to the cosine of the phase angle between voltage and current. If the load were purely reactive, either capacitive or inductive, the PF is zero. This means that no real is power is dissipated as the reactive component receives power but later returns the power. This is, of course, an illustrative example as there will always be some power dissipation in the actual components in a circuit, e.g., diodes, switches, transformers, etc. However, even in the illustrative example, if no real power were dissipated, there is real current and real voltage signals involved. So, if a circuit breaker carries the real current in the powerless case, the breaker may still blow. This is the reason that loads on a circuit breaker with reactive components will not be able to draw as much power as a resistive load. For example, a resistive load presents a PF of 1, but any load with a power factor less than one will draw power reduced by the power factor. So a resistive load might draw a maximum current (say, to not trip a breaker) from a line, but a load with a PF of 0.5 would only be able to draw ½ the maximum power and still not trip a circuit breaker. One way of expressing the issue of PF, is that the power available from an AC circuit is:
Pout=(V)(I) (PF)(E). Where the V and I are rms values, and where E is a measure of the power loss of any connecting circuitry.
Another issue with reactive loads is that, when the stored power is returned to the original source, say typically to the AC line or mains socket, the power company and other users on the same line must be able to handle it. But, if the returned power includes the harmonic and non-linear current of a low power factor system, there may be related problems and unusual behavior for other users of the same power source.
When the AC load is a switch-mode power supply, the current drawn is non-linear with a power factor typically of about 0.65. The current pulses are short, discontinuous pulses. Besides being careful of tripping circuit breakers, such non-linear current pulses produce harmonics that contribute to noise and to unwanted heating of the connecting circuitry, wiring, etc. It should be noted that the 0.65 PF of a typical “capacitor input” switch-mode power supply is created primarily by the harmonic currents and not by the phase angle between the fundamental (sinusoidal) line voltage and current. In this case, the ampere term in the denominator of the power factor equation is in the formIac RMS=√{square root over (Ifund2+I3rd2+I5th2+ . . . )}where Ifund is the fundamental frequency and I3rd, I5th and etc. represent each of the current harmonics.
Switch-mode power supplies routinely include PFC circuitry to overcome the above problems, and, maybe more importantly, harmonic content fed into power sources are limited by government regulatory agencies.
Reiterating, known PFC switch-mode power supplies with a PFC controller include current and voltage sensing that dissipate too much power and are bulky.